Radio transmitter antennae comprising a plurality of open-ended coaxial cavities and means for exiting them with pulsed electron beams

ABSTRACT

A radio antenna suitable for the transmission of high-power signals in the medium and high radiofrequency ranges, includes at least one electromagnetically resonant cavity, each cavity being excited by at least one pulsed electron beam which interacts with the cavity and generates radiofrequency oscillations within it. A radiator element is mounted on and directly connected to the cavity. Embodiments comprising two cavities and modulation apparatus, for modulating the signals transmitted by controlling the relative phases of the electron beam pulses, are described.

United States Patent Inventors Harold Kilner Robin 17 Broadwater Down,Tunbridge Wells,

Kent; Francis Michael Russell.

130 Oxford Road. Abingdon.

Berkshire, both of England 738,335

June 19, 1968 Aug. 3, 1971 June 19, 1967 Great Britain Appl. No. FiledPatented Priority RADIO TRANSMITTER ANTENNAE COMPRISING A PLURALITY OFOPEN-ENDED COAXIAL CAVITIES AND MEANS FOR EXI'IING THEM WITH PULSEDELECTRON BEAMS 13 Claims, 5 Drawing 1 1;.

1-1. c1 1104b 1/04 Field of Search 325/120,

[56] Reierencm Cited UNITED STATES PATENTS 2,406,370 8/ I 946 Hansen etal. 325/120 3,080,523 3/1963 Miller 325/120 3,098,980 7/1963Doclington... 325/120 3,119,965 1/1964 Phillips 325/120 3,292,03312/1966 Kenmoku.... 325/120 3,473,125 10/1969 Babillon 325/120 PrimaryExaminer- Robert L. Griffin Assistant Examiner-Albert J. MayerAltorneyCameron, Kerkam & Sutton FROM 55 1 5011 53 59 57 07 7| 7p 66 S6S8 aee'nsu-zn TELEMETRY TELEMETRY v RECHFIER 1 I "men ramsm'r-rearnmsmrren MV FILTER l I l BEAM BEAM CONTROL umr CONTROL UNIT (RECEIVER a(RECEIVER a 130 POWER AMP) POWER AMP) SUBSIDIARY SUBSIDIARY COLLECTOR(consume) 7 ELECTRODE ELECTRODE 8 [1? iii D a 8c:

SUPPLY PATENTEUAUG sum 3,597,691

sum 1 0F 5 w d W 2 PATENTEU-Aus 3|97l 3 597,591

I sum 2 BF 5 PATENTED ms 31% SHEET 0F 5 FIG.

PATENTEU IIuI; 3 IHYI I SHEET 5 UF 5 I I FROMSS FROM 54 I 5,9 511 67 7|7;) 66 ls gg I RECTIFIER V L TELEMETRY TELEMETRY' v RECTIFIER I I&F|LTER *Mv TRANSMITTER TRANSMITTER Mv 8.F|LTER I III 1' BEAM BEAM flabCONTROL UNIT CONTROL UNIT (RECEIVER & (RECEIVER &

POWER AMP) POWER AMP) SUBSIDIARY SUBSIDIARY CQLLECTOR COLLECTOR 7ELECTRODE ELECTRODE 8 i -I- I I! I I Q51 "v' E.H.T.

SUPPLY FIGS The present invention relates to radio antennae of a kindparticularly suitable for the transmission of large quantities of powerin the medium and high radiofrequency ranges. The inefficiencies ofknown methods of generating radiofrequency power, of coupling it to anantenna and radiating it as an electromagnetic wave are the fundamentalcauses of most of the difficulties and costs encountered in thetransmission of highpower radiofrequency signals. These inefficienciesare not only wasteful but troublesome, since they tend to cause thegeneration of considerable quantities of heat at places where heat isnot wanted, and where overheating must be avoided.

It is known from the work of Haeff and Nergaard published in theProceedings of the IRE, Mar. I940, pp. l26l30, that the interaction ofapulsed electron beam with an electromagnetically resonant cavity mayafford an efficient means for transforming direct-current power intoradiofrequency power.

According to the present invention, a radio transmitter antenna includesan electromagnetically resonant cavity, a radiator element mounted onand directly connected to the cavity, and excitation means for providingat least one pulsed electron beam to interact with the cavity andgenerate radiofrequency oscillations therein. The cavity may be aquarter-wave coaxial cavity, and the radiator element may be anextension of the center conductor of the coaxial cavity. The excitationmeans may include one or more electron beam accelerator-deceleratortubes connected between the inner and outer conductors of the coaxialcavity.

By making the antenna in the form of a resonant cavity and exciting thecavity directly with one or more pulsed electron beams, the usual needfor the transmission of the output radiofrequency power from atransmitter through transmission lines and coupling devices is avoided.In effect, the final stage of the transmitter is mounted within theantenna, and the antenna provides the tuned load for the finaltransmitter stage. This reduces the number of components required tocarry high radiofrequency power and thereby increases the efficiency ofthe arrangement, reducing both the capital costs and the running costsinvolved.

The energy supplied to a single-cavity arrangement may be modulated bycontrolling the amplitude or the duration (or both) of the beam pulses.This is satisfactory for on/off code transmissions such as telegraphyand frequency-shift-keyed transmissions. While modulation by an analoguesignal is possible in a system of this kind, it is comparativelyinefficien't unless the high acceleration voltage can be modulated aswell, which would introduce additional difficulties and complications.

To provide a convenient and efficient arrangement for the amplitudemodulation of the radiofrequency power output according to an analoguesignal, the antenna may include a plurality of electromagneticallyresonant cavities, each having a radiator element mounted on andconnected to it and each having separately controllable excitation meansfor providing at least one pulsed electron beam to interact with thecavity and generate radiofrequency oscillations therein. The cavitiesmay be quarter-wave coaxial cavities with their radiator elements closeto each other. The radiator elements may be extensions of the centerconductors of the cavities. The excitation means may include one or moreelectron beam accelerator-decelerator tubes in each cavity, connectedbetween the inner and outer conductors of the cavity. The electron beamsofthe accelerator-decelerator tubes are controllable by applying voltagepulses to grid or modulator electrodes. Stabilizing means may beprovided for stabilizing the amplitude of the oscillations, and mayinclude a feed back arrangement responsive to the induced radiofrequencyvoltage and controlling the duration of the beam pulses.

To generate and transmit radiofrequency power, the electron beams arepulsed under the control of signals applied'to the modulator electrodes.The beam pulses are arranged to have a recurrence frequency equal to thefrequency of the radio wave which is to be generated, and the cavitiesare tuned to resonate at this frequency. In each cavity the electronbeams induce radiofrequency oscillations which are synchronized with andwill interact with the beam pulses. The radiofrequency voltage inducedin the cavity decelerates the electrons and power is transferred fromthe beam to the radiofrequency electromagnetic field; this power isderived from a direct voltage power supply which is provided toaccelerate the electron beams before they enter the cavity.

The electron beam pulses occur once in each cycle of the radiofrequency, and have a duty cycle of less than or not much more than sixpercent. Hence, they will extend for a time which corresponds to a phaseangle of not more than I 1 degrees on each side of the occurrence ofpeak radiofrequency voltage across the deceleration section of thetubes.

To provide a modulated radiofrequency output, the timing of the beampulses may be controlled to induce radiofrequency signals of stabilizedamplitude in different cavities which are out of phase with each otherby variable but controlled amounts. The waves radiated from the cavitiesthen combine vectorially to produce a resultant whose amplitude dependson the relative phase displacements between the signals; its amplitudecan be modulated by varying the relative phase displacement of thepulses in different cavities. For example, there may be two cavitiessupplied with push-pull pulse-position-modulating signals.

The axes of the coaxial cavities may be parallel to each other, andtheir open ends and radiator elements may be covered by a radome. Theradome may incorporate or support a plurality of conductive annuliarranged in planes normal to the axes of the cavities. This ensures thatthe waves radiated will be polarized in a direction parallel to the axesof the cavities.

An embodiment of the invention will not be described, by way of exampleonly, with reference to the accompanying drawing, of which:

FIGS. 1 and 2 are sectional elevations of two alternative high-powertransmitting aerial assemblies,

FIG. 3 is a schematic block diagram of modulation apparatus for use inthe aerial assembly of FIG. 1 or FIG. 2,

FIG. 4 is a graphical representation of waveforms illustrating theoperation of the modulation apparatus of FIG. 3, and

FIG. 5 is a schematic block circuit diagram showing the connections ofthe modulation apparatus of FIG. I to the tubes and cavities of theaerial assemblies.

FIG. 1 shows in cross section two similar resonant cavities 1 and 2,formed of quarter-wavelengths of coaxial line, arranged close togetherwith their axes vertical, and short circuited at their lowest ends. Theupper 3, 4 of the resonant cavities are open, and radiator elements 3, 4are mounted on top of their inner conductors 5, 6 respectively. A numberof electron beam accelerator-decelerator tubes are mounted radiallyacross each cavity; two of these are shown and marked 7, 8. The outerconductors 9, 10 of the cavities 1 and 2 are electrically connectedtogetherfat least at their upper and lower ends. The open ends of thecavities are covered by a hemispherical radome 11 of low loss dielectricmaterial. The structure of the radome 11 incorporates a plurality ofseparate horizontal conductive annuli 12.

Each of the electron beam accelerator-decelerator tubes has an electrongun at one end and a water-cooled collector at the other. The electronguns each include a modulator or grid electrode, enabling their electronbeams to be pulsed under the control of a comparatively low voltagesignal. Each tube has an acceleration region between its electron gunand a principal and a deceleration region between its principal anodeand its collector. The tubes also have subsidiary collector electrodes7a, 8a. The decelerating regions cross the resonant cavities radially,whereas the acceleration regions are screened from the radiofrequencyfields within the cavities. The tubes are fitted with magnet coils (notshown) to counteract space-charge repulsion effects which tend to makethe electron beams spread excessively especially in the decelerationregion.

In the embodiment of FIG. 1, the electron guns and acceleration regionsof the accelerator-decelerator tubes are mounted within the extensionsof the inner conductors 5, 6 which form the radiator elements 3, 4. Highdirect voltage acceleration power supplies are conducted to the tubesthrough cables (not shown) within the inner conductors 5,6.

A vertical conductive electrostatic screen 13, connected to the outerconductors 9, I0, is provided to reduce the capacitance between theradiator elements 3 and 4. Adjustable sets of copper tapes l5, 16 arefestooned and connected between the inner and outer conductors ofthecavities l and 2 respectively, at their lower ends. Beam control unitsare mounted within the radiator elements 3 and 4 and connected to themodulator electrodes of the accelerator-decelerator tubes. The radiatorelement 4 is shown partially cut away to reveal one of the beam controlunits, referenced 18. The beam control unit are controlled by amodulator 19. Signals from the modulator 19 are conveyed to the beamcontrol units in the form of modulated beams of infrared light viamirrors 20 and 21 respectively, along paths indicated by thechain-dotted lines 27, 28. The line 24 represents ground level.

The aerial is intended to operate at approximately 1400 kHz.; this isthe resonant frequency of the cavities l and 2, which are about 30 feetin diameter and I60 feet high. The high direct voltage applied betweenthe principal anodes and the electron guns of theaccelerator-decelerator tubes is about one million volts. The electronbeams are pulsed at the operating frequency of 1400 kHz.; each pulse hasa duration equivalent to a phase angle of the order of IO to 20 degreesat this frequency. The pulsed beam currents induce radiofrequencyvoltages in the two cavities which decelerate the electron beam and drawenergy from them. The beam pulses are controlled by the signals from themodulator 19 so that the induced radiofrequency voltages in the twocavities l and 2 are in antiphase when a modulation signal has a minimumvalue, and have a relative phase displacement at other times which isdecreased as the modulation signal increases. In each of the cavities Iand 2, a feedback arrangement is provided which is responsive to theinduced radiofrequency voltage and which controls the duration of thebeam pulses, thus stabilizing the amount of energy fed to each cavityand thereby stabilizing the induced radiofrequency voltage therein sothat its peak amplitude is somewhat less than the direct voltage appliedacross the acceleration regions of the acceleratordecelerator tubes.This is achieved by connections 25, 26 from subsidiary collectorelectrodes 7a in 8a in the acceleratordecelerator tubes to the modulator19. The subsidiary collector electrodes draw currents which arecritically dependent on the spreading of the electron beams. The beamspreading is in turn critically dependent on the difference between thedirect voltage and the induced radiofrequency voltage. Hence thecavities are made to produce signals of equal amplitudes, which combinevectorially to give a resultant whose amplitude is determined by therelative phase displacement of the beam pulses in the two cavities. Theresultant transmitted wave is therefore amplitude modulated inaccordance with the modulation signal from which the relativedisplacement is derived. The festoons of copper tape 15, 16 are adjustedto make the cavities 1 and 2 resonant at the desired carrier frequency.The coupling between the cavities is adjusted by alterations to thescreen 13 to achieve a desired bandwidth, preferably with transitionalcoupling.

The radiation from the radiator elements causes the exter nal surfacesofthe outer conductors 9, 10 of the cavities to be excited like avertical monopole, but with a desirably high impedance and Q-factor.However, the horizontal separation of the radiator elements 3, 4 would,in the absence of the conductive annuli l2, superimpose the effect ofasmaller horizontal dipole on the radiated field. The conductive annuli12 short circuit the horizontal field components.

Harmonics of the desired frequency can be reduced by arranging that theimpedance presented at the acceleratordecelerator tubes by the cavitiesto unwanted harmonics is low or as near as possible to a short circuit.This may be achieved by mounting suitable conductive sleeves (not shown)over the inner conductors 5 and 6. The sleeves may each be arranged toproduce a standing wave null, or to act as a series-tuned short circuitat a harmonic frequency. Alternatively, the lowest of the conductiveannuli 12 (marked 12a in the drawing) may be connected by an inductance(not shown) to the outer conductors 9, 10 or to earth so that it acts asa seriestuned short circuit at the harmonic frequency. Yet anotherpossible method for suppressing harmonics would be to initiatesubsidiary beam pulses in one or more of the accelerator-deceleratortubes, controlled in timing and duration so that they exactly counteractany harmonic frequency components ofthe signals induced by the main beampulses.

Various modifications of the arrangement of FIG. 1 are possible. Forinstance, the accelerator-decelerator tubes could be all mounted theother way round, with their collectors connected to the inner conductorsand their electron guns and acceleration regions outside the outerconductors. A better arrangement still has two sets of tubes in eachcavity, with alternate tubes facing opposite ways and being pulsed inantiphase with each other. This doubles the duty cycle of the powerinput, enabling a given power to be obtained with half the acceleratingvoltage needed in an equivalent simple system. The impedance required tothe cavities is reduced, and the bandwidth ofthe antenna increased.

Instead of being mounted radially across the open ends of the cavities land 2, the accelerator-decelerator tubes may be mounted vertically in asquirrel cage formation on top of the cavities.

FIG. 2 shows an embodiment of this type. Parts of the transmitterassembly of FIG. 2 which correspond to similar parts in the transmitterof FIG. 1 are given the same reference numbers in both drawings, andneed not again be described in detail.

FIG. 2 shows sections of two similar resonant cavities l and 2 formed,as in FIG. 1, of quarter wavelengths of coaxial line arranged closetogether with their axes vertical and short circuits at their lowerends. The accelerator-decelerator tubes are connected to a high-voltagesupply 41 through high-voltage leads 38,39 which are supported withinthe inner conductors 5, 6 and the spaces around them are pressurizedwith nitrogen or sulphur-hexaflouride. They are connected to ahigh-voltage power supply unit 41 which is immersed in oil in a tank 40.At the open ends of the cavities 1 and 2 are conductive annuli 34 and 35which connect squirrel cage formations of accelerator-decelerator tubes7, 8 supported vertically on disc-shaped enlargements 30, 31 of theinner conductors 5, 6. The inner conductors 5, 6 are braced from theouter conductors 9, 10 by a plurality of radial quartz rods 36, 37.

Above the disc-shaped enlargements 30, 31 are the aforementionedradiator elements 3, 4.

Beam control units 18a, 18b are contained within the radiator elements3, 4 and modulator units 19a, 19b are contained within the conductiveannuli 34, 35. The remaining structure is substantially similar to theembodiment of FIG. 1 already described, and the aerial assembly isoperated in a similar way. Suitable pulse-timing circuits for performingthe modulating and stabilizing functions into either of theabove-described embodiments will now be described in greater detail withreference to FIGS. 3 and 4.

FIG. 3 shows a pulse generator 51 with an output connected to inputs oftwo fixed monostable multivibrators 52 and 53. Outputs from themultivibrators 52 and 53 are connected to inputs of monostablemultivibrators 54 and 55 respectively. Outputs from the multivibrators54 and 55 are connected to inputs of monostable multivibrators 56 and 57respectively. The multivibrators 54, 55, 56 and 57 are variable, beingconstructed to produce pulses of duration dependent on a voltage appliedto control inputs with which they are provided. The control inputs ofthemultivibrators 56 and 57 are connected through rectifier and filterunits 58, 59 respectively to the lines 25, 26 respectively, whichreceive signals from the subsidiary collector electrodes of theaccelerator-decelerator tubesin the cavities 1 and 2 respectively. Thecontrol inputs of the multivibrators 54 and 55 are both connected to amodulation input line 60. The multivibrators 56, 57 have output lines66, 67 respectively.

Typical signals from the outputs of the above-described units 51, 52,54, 56, 53, 55 and 57 are represented graphically on a common horizontaltime scale at (a), (b), (c), (d), (e), (j), and ('g) respectively ofFIG. 4.

The pulse generator 51 generates a pulse train (shown at (a) in FIG. 4)having a recurrence period equal to the period of the radiofrequencysignal which is to be transmitted by the aerial, and at which thecavities l and 2 are adjusted to resonate. This period is indicated inFIG. 4 and hereinafter by the symbol T. The trailing edge of each pulsefrom the pulse generator 51 is applied to trigger the multivibrators 52and 53. The multivibrator 52 produces pulses of duration T/2 as shown at(b) in FIG. 4; the trailing edges of these pulses are applied to triggerthe multivibrator 54, causing it to produce pulses as shown at (c) inFIG. 4. The duration of each pulse produced by the multivibrator 54 iscontrolled by a modulation voltage Vm applied on the line 60 so that itis substantially equal to A;T+l Vm-V0) but never exceeds /zT, where k isa constant and V0 is the minimum value of the modulation voltage. Thetrailing edge of each pulse from the multivibrator 54 triggers themultivibrator 56, causing it to produce a pulse as shown at(d) in FIG.4.v

The multivibrator 53 produces pulses of duration equal to 3T/4, as shownat (e) in FIG. 4. The trailing edges of these pulses are applied totrigger the multivibrator 55, causing it to produce pulses as shown at(f) in FIG. 4. The duration of each pulse produced by the multivibrator55 is controlled by the modulation voltage Vm so that it issubstantially equal to VzT- k( VmVc), and never less than AT. Thetrailing edge of each pulse from the multivibrator 55 triggers themultivibrator57 causing it to produce a pulse as shown at (g) in FIG. 4.

To achieve the desired relationship between the modulation voltage andthe durations of the pulses produced by the variable multivibrators 54,55 an inverting amplifier (not shown) may be connected in series withthe control input to one of the multivibrators 54 or 55, oralternatively their control inputs may be fed from push-pull modulationinputs.

The result of the actions described above is therefore to generate twopulse trains as shown at (d) and (g) in FIG. 4, each having a pulserecurrence frequency equal to the radio frequency to be transmitted andhaving a'phase displacement relative to each other which decreasesproportionately as the modulation voltage increases. After suitableamplification in the beam control units 18, one of these pulse trains isused to switch on the electron beams in the tubes of one of the resonantcavities, and the other pulse train is used to switch on the electronbeams in the accelerator-decelerator tubes of the other resonant cavity.This induces radiofrequency voltages which are radiated as hereinbeforedescribed. As the modulating voltage V!" increases from its minimumvalue the amplitude of the resultant formed by the vectorial combinationof the radiated waves increases having a relationship to the modulationvoltage which is substantially linear over a useful range.

A feedback signal, derived from the currents drawn by the subsidiarycollector electrodes of all the accelerator-decelerator tubes whosebeams are controlled by the pulses from the multivibrator 56, isrectified and smoothed in the rectifier and filter unit 58 and appliedto control the duration of each pulse produced by the multivibrator 56,so as to stabilize the power input to and the radiofrequency powerdeveloped in the associated cavity. Another feedback signal similarlyderived from the tubes in the other cavity, is rectified and smoothed inthe rectifier and filter units 59 and applied to control the duration ofeach pulse from the multivibrator 57, to stabilize the power in theother cavity.

The units 52 to 57 inclusive of FIG. 3 may be incorporated in themodulator unit 19 (FIG. 1) or divided between the modulator units 19a,1912 (FIG. 2). The rectifier and filter units'58 and 59 may be mountedclose to the collector ends of the accelerator-decelerator tubes or inthe modulator unit 19 or the modulator units 19a, 19b Bearing in mindthat in the operation of the aerial there is expected to be a directvoltage of about 500 kilovolts between the cavity inner conductors 5, 6and the electron guns and modulator connections of theaccelerator-decelerator tubes, and a radiofrequency voltage of about 500kilovolts r.m.s. between the radiator elements 3, 4 and the upper endsof the outer conductors 9, 10, it will be understood that thetransmission of the signals on at least two of the connection linesindicated in FIG. 3 may involve some conventional form of telemetry, forexample an infrared link as hereinbefore described in connection withFIG. 1. FIG. 5 shows the connections required. In the modulator 19 theoutputs 66, 67 from the multivibrators 56, 57 are applied toconventional telemetry transmitters 70, 71 which transmit the modulatoroutput signals, in the form of pulses of infrared light or otherradiation, along the paths 27, 28 to the beam control units 18b, 18arespectively. The beam control units 18a, 18b comprise conventionalreceivers and amplifiers for receiving the signals and reconverting theminto electrical form and to a suitable voltage for pulsing the electronbeams in the accelerator-decelerator tubes 7 and 8, respectively. Thesubsidiary collector electrodes 7a, 8a of the acceleratordeceleratortubes 7 and 8 are connected by lines 25, 26 to the rectifier and filterunits 59, 58 which control the periods of the multivibrators 57, 56,respectively. Since the pulse-timing cir cuits (51 to 55) in themodulator unit 19 have already been fully described with respect to FIG.3, it is unnecessary to describe them again and they are not fully shownin FIG. 5. The deceleration regions of the tubes 7, 8 are indicated at7d, 8d, respectively. The electrical connections formed by the innerconductors 5, 6 and the outer conductors 9, 10 of the cavities areindicated diagrammatically and out of proportion .in FIG.5.

suitably phased pulse trains for controlling the electron beams. If twosets of tubes are used in each cavity as hereinbefore suggested, themodulation system will have to be elaborated to provide one pulse trainfor each set of accelerator-decelerator tubes. Some pulse-timingcircuits comprising multivibrators as shown in FIG. 3 may then bemounted in or adjacent to the units 18. The cavities may be made ascompartments formed by one or more vertical subdivisions in an uprightcylindrical structure, for example with an elliptical cross sectionsubdivided along its minor axis. The acceleratordecelerator tubes may beair-cooled or water-cooled. In place of the infrared telemetry system,the output signals from the multivibrators 56, 57 could be passedthrough suitable isolating transformers in the power supply tank 41(FIG. 2) and transmitted by cables up the center of the high-voltageleads 38, 39 to the beam control units 18a, 18b. In this case the beamcontrol units would clearly not need to include telemetry receivers andthey could be simple amplifiers.

Iclaim:

I. A radio transmitter antenna comprising two tunable open-endedquarter-wave coaxial electromagnetically resonant cavities both tuned tothe same resonant frequency, each having an inner conductor and an outerconductor and disposed with their axes parallel and their open endsadjacent to each other,

a plurality of radiator elements, comprising one radiator elementmounted on the inner conductor of each of the said resonant cavities andprotruding from the open end thereof,

a plurality of accelerator-decelerator tubes, comprising at least oneaccelerator-decelerator tube for each one of the said resonant cavities,each tube having an evacuated envelope, a modulator electrode and adeceleration region within said envelope and between two furtherelectrodes with external electrical connections, and being coupled toone only of the said cavities and being mounted thereon with theelectrode at one end of its deceleration region electrically connectedto the inner conductor of the cavity and the electrode at the other endof its deceleration region electrically connected to the outer conductorof the cavity,

means for applying a high-voltage power supply to each of the saidaccelerator-decelerator tubes,

pulse generator means for generating a first train of pulses having arecurrence frequency equal to the resonant frequency of said cavities,

further means, connected to the said pulse generator means and having amodulation input connection, for deriving a second train of pulses and athird train ofpulses both having the same recurrence frequency as thesaid first train of pulses but being phased relative to each other sothat the phase difference between the said second train and the saidthird train is dependent on a modulation voltage applied to the saidmodulation input connections and will be substantially half a period ofthe recurrance frequency when the said modulation voltage has apredetermined minimum value and will decrease whenever the modulationvoltage increases,

and means for applying the second train of pulses to theaccelerator-decelerator tube or tubes in one of the said two cavitiesand for applying the third train of pulses to theaccelerator-decelerator tube or tubes in the other of the said twocavities.

2. A radio transmitter antenna is claimed in claim 1 and wherein thesaid further means comprises a first monostable multivibrator circuitconnected to the pulse generator means, and constructed to producepulses of duration equal to half a period at the recurrance frequency,

a second monostable circuit connected to the pulse generator means andconstructed to produce pulses of duration equal to three-quarters of aperiod at the recurrence frequency,

a third monostable multivibrator circuit connected to the firstmultivibrator circuit and to the modulation input connection, andconstructed to produce pulses of duration substantially linearlyincreasing with the modulation voltage,

and a fourth monostable multivibrator circuit connected to the secondmultivibrator circuit and constructed to produce pulses of durationsubstantially linearly related to the modulation voltage but decreasingas the modulation voltage increases. i

3. A radio transmitter antenna as claimed in claim 2 and comprising afirst detector means mounted in the said one of the said two cavitiesfor deriving a first feedback signal indicative of the amplitude ofradiofrequency signals developed therein,

a second detector means mounted in the said other of the said twocavities for deriving a second feedback signal indicative of theamplitude of radiofrequency signals developed therein, fifth monostablemultivibrator circuit connected to the said first detector means and tothe said third monostable multivibrator circuit and constructed toproduce pulses of duration controlled by the said first feedback signal,and a sixth monostable multivibrator circuit connected to the saidsecond detector means and to the said fourth monostable multivibratorcircuit and constructed to produce pulses of duration controlled by thesaid second feedback signal,

and wherein the output of the fifth monostable multivibrator circuit isconnected to the modulator electrode of the accelerator-decelerator tubeor tubes in the said one of the resonant cavities, and the output of thesixth monostable multivibrator circuit is connected to the modulatorelectrode of the accelerator-decelerator tube or tubes in the said otherof the resonant cavities.

4. A radio transmitter antenna as claimed in claim 1, and wherein aplurality of accelerator-decelerator tubes are mounted radially acrossthe open end of each of the resonant cavities.

5. A radio transmitter antenna as claimed in claim 1 wherein each of theresonant cavities has an outer conductor formed with an inwardlyextending annular lip at its open end, and an inner conductor formedwith a disc like enlargement near the open end of the cavity, andwherein a plurality of accelerator-decelerator tubes are mountedsubstantially longitudinally in each cavity with their decelerationregions between the said annular lip and the said disc like enlargement.

6. A radio transmitter antenna as claimed in claim 1 and comprising aradome covering the said resonant cavities and the said radiatorelements.

7. A radio transmitter antenna as claimed in claim 6 and comprising aplurality of conductive annuli mounted on the radome so that eachannulus lies in a plane normal to the axes of the resonant cavities.

8. A radio transmitter antenna as claimed in claim 1 and comprising anelectromagnetic screening element mounted between the radiator elements.

9. A radio transmitter antenna comprising a plurality of tunableopen-ended quarter-wave coaxial electromagnetically resonant cavities,each having an inner conductor and an outer conductor, and all disposedwith their axes parallel and their open ends adjacent to each other,plurality of radiator elements, comprising one radiator element mountedon and electrically connected to the inner conductor of each of the saidresonant cavities and protruding from the open end thereof, andexcitation means connected to the said cavities for directing at leastone separate electron beam across each of the said cavities so that eachelectron beam crosses one only of the said cavities and causing theelectron beam to be pulsed with a repetition rate related to thefundamental resonant frequency of the cavities,

the said excitation means comprising pulse-timing control means having amodulation input connection for causing a pulse position modulation ofthe pulses of the electron beams so that there will be a differencebetween the timing of the electron beam pulses in one of the cavitiesand the timing of the electron beam pulses in another of the cavities,and for varying the magnitude of the said difference in response tosignals applied to the said modulation input connection.

10. A radio transmitter antenna comprising a plurality of tunableopen-ended quarter-wave coaxial electromagnetically resonant cavities,each having an inner conductor and an outer conductor, and all disposedwith their axes parallel and their open ends adjacent to each other,

a plurality of radiator elements, comprising one radiator elementmounted on the inner conductor of each of the said resonant cavities andprotruding from the open end thereof,

plurality of accelerator-decelerator tubes, comprising at least oneacceleratondecelerator tube for each one of the said resonant cavities,each tube having an evacuated envelope, a modulator electrode and adeceleration region within said envelope and between two furtherelectrodes with external electrical connections, and being coupled toone only of the said cavities and being mounted thereon with theelectrode at one end of itsdeceleration region electrically connected tothe inner conductor of the cavityand the electrode at the other'end ofits deceleration region electrically connected to the outer conductor ofthe cavity,

means for applying a high-voltage power supply to each of pulse positionmodulation of the pulses of the electron beams so that there will be adifference between the timing of the beam pulses in one of the saidcavities and the timing of the beam pulses in another of the saidcavities, and for varying the magnitude of the said difference inresponse to signals applied to the said modulation input connection.

ll. A radio transmitter antenna as claimed in claim 10 and comprisingstabilization means, connected to each of the said cavities andresponsive to the amplitude of radiofrequency oscillations inducedtherein by the electron beam pulses, and connected to the modulatormeans, for controlling the energy of the electron beam pulses so as tostabilize the said amplitude.

12. A radio transmitter antenna as claimed in claim 11 and wherein thesaid stabilization means comprises a detector means mounted in eachofthe said cavities for deriving a feedback signal indicative of theamplitude of radiofrequency oscillations induced therein, andpulse-width-controlling means connected to receive the said feedbacksignal from the said detecting means and connected to the saidmodulatormeans for controlling the duration of the electron beam pulses.

13. A radio transmitter antenna as claimed in claim 12 and wherein thesaid detector means comprises a subsidiary collector electrode in eachof the said accelerator-decelerator tubes.

72 8? UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No.3'597'69l D t d August 1971 Inventor) Harold Kilner Robin and FrancisMichael Russell It is certified that error appears in theabove-identified patent and that said Letters Patent are herebycorrected as shown below:

7 In the title cancel "exiting" and insert -exciting--. I Column 2, line39, cancel "not" and insert -now-,- line 55, cancel "3,4" and insert--ends--; line 72, after "principal" insert -anode-. Column 3, line 23,cancel "unit" and insert --units-. Column 4, line 66, cancel "into" andinsert --in-. Colunm 5,

line 29, cancel "Vm" and insert --V line 40, cancel "Vc" and insertVo--.

Signed and sealed this 28th day of March 1972.

(SEAL) \ttest:

EDWARD M.FLETCH ER,JR. ROBERT GOI'TSCHALK Ittesting Officer Commissionerof Patents

1. A radio transmitter antenna comprising two tunable open-endedquarter-wave coaxial electromagnetically resonant cavities both tuned tothe same resonant frequency, each having an inner conductor and an outerconductor and disposed with their axes parallel and their open endsadjacent to each other, a plurality of radiator elements, comprising oneradiator element mounted on the inner conductor of each of the saidresonant cavities and protruding from the open end thereof, a pluralityof accelerator-decelerator tubes, comprising at least oneaccelerator-decelerator tube for each one of the said resonant cavities,each tube having an evacuated envelope, a modulator electrode and adeceleration region within said envelope and between two furtherelectrodes with external electrical connections, and being coupled toone only of the said cavities and being mounted thereon with theelectrode at one end of its deceleration region electrically connectedto the inner conductor of the cavity and the electrode at the other endof its deceleration region electrically connected to the outer conductorof the cavity, means for applying a high-voltage power supply to each ofthe said accelerator-decelerator tubes, pulse generator means forgenerating a first train of pulses having a recurrence frequency equalto the resonant frequency of said cavities, further means, connected tothe said pulse generator means and having a modulation input connection,for deriving a second train of pulses and a third train of pulses bothhaving the same recurrence frequency as the said first train of pulsesbut being phased relative to each other so that the phase differencebetween the said second train and the said third train is dependent on amodulation voltage applied to the said modulation input connections andwill be substantially half a period of the recurrance frequency when thesaid modulation voltage has a predetermined minimum value and willdecrease whenever the modulation voltage increases, and means forapplying the second train of pulses to the accelerator-decelerator tubeor tubes in one of the said two cavities and for applying the thirdtrain of pulses to the accelerator-decelerator tube or tubes in theother of the said two cavities.
 2. A radio transmitter antenna isclaimed in claim 1 and wherein the said further means comprises a firstmonostable multivibrator circuit connected to the pulse generator means,and constructed to produce pulses of duration equal to half a period atthe recurrance frequency, a second monostable circuit connected to thepulse generator means and constructed to produce pulses of durationequal to three-quarters of a period at the recurrence frequency, a thirdmonostable multivibrator circuit connected to the first multivibratorcircuit and to the modulation input connection, and constructed toproduce pulses of duration substantially linearly increasing with themodulation voltage, and a fourth monostable multivibrator circuitconnected to the second multivibrator circuit and constructed to producepulses of duration substantially linearly related to the modulationvoltage but decreasing as the modulation voltage increases.
 3. A radiotransmitter antenna as claimed in claim 2 and comprising a firstdetector means mounted in the said one of the said two cavities forderiving a first feedback signal indicative of the amplitude ofradiofrequency signals developed therein, a second detector meansmounted in the said other of the said two cavities for deriving a secondfeedback signal indicative of the amplitude of radiofrequency signalsdeveloped therein, a fifth monostable multivibrator circuit connected tothe said first detector means and to the said third monostablemultivibrator circuit and constructed to produce pulses of durationcontrolled by the said first feedback signal, and a sixth monostablemultivibrator circuit connected to the said second detector means and tothe said fourth monostable multivibrator circuit and constructed toproduce pulses of duration controlled by the said second feedbacksignal, and wherein the output of the fifth monostable multivibratorcircuit is connected to the modulator electrode of theaccelerator-decelerator tube or tubes in the said one of the resonantcavities, and the output of the sixth monostable multivibrator circuitis connected to the modulator electrode of the accelerator-deceleratortube or tubes in the said other of the resonant cavities.
 4. A radiotransmitter antenna as claimed in claim 1, and wherein a plurality ofaccelerator-decelerator tubes are mounted radially across the open endof each of the resonant cavities.
 5. A radio transmitter antenna asclaimed in claim 1 wherein each of the resonant cavities has an outerconductor formed with an inwardly extending annular lip at its open end,and an inner conductor formed with a disc like enlargement near the openend of the cavity, and wherein a plurality of accelerator-deceleratortubes are mounted substantially longitudinally in each cavity with theirdeceleration regions between the said annular lip and the said disc likeenlargement.
 6. A radio transmitter antenna as cLaimed in claim 1 andcomprising a radome covering the said resonant cavities and the saidradiator elements.
 7. A radio transmitter antenna as claimed in claim 6and comprising a plurality of conductive annuli mounted on the radome sothat each annulus lies in a plane normal to the axes of the resonantcavities.
 8. A radio transmitter antenna as claimed in claim 1 andcomprising an electromagnetic screening element mounted between theradiator elements.
 9. A radio transmitter antenna comprising a pluralityof tunable open-ended quarter-wave coaxial electromagnetically resonantcavities, each having an inner conductor and an outer conductor, and alldisposed with their axes parallel and their open ends adjacent to eachother, a plurality of radiator elements, comprising one radiator elementmounted on and electrically connected to the inner conductor of each ofthe said resonant cavities and protruding from the open end thereof, andexcitation means connected to the said cavities for directing at leastone separate electron beam across each of the said cavities so that eachelectron beam crosses one only of the said cavities and causing theelectron beam to be pulsed with a repetition rate related to thefundamental resonant frequency of the cavities, the said excitationmeans comprising pulse-timing control means having a modulation inputconnection for causing a pulse position modulation of the pulses of theelectron beams so that there will be a difference between the timing ofthe electron beam pulses in one of the cavities and the timing of theelectron beam pulses in another of the cavities, and for varying themagnitude of the said difference in response to signals applied to thesaid modulation input connection.
 10. A radio transmitter antennacomprising a plurality of tunable open-ended quarter-wave coaxialelectromagnetically resonant cavities, each having an inner conductorand an outer conductor, and all disposed with their axes parallel andtheir open ends adjacent to each other, a plurality of radiatorelements, comprising one radiator element mounted on the inner conductorof each of the said resonant cavities and protruding from the open endthereof, a plurality of accelerator-decelerator tubes, comprising atleast one accelerator-decelerator tube for each one of the said resonantcavities, each tube having an evacuated envelope, a modulator electrodeand a deceleration region within said envelope and between two furtherelectrodes with external electrical connections, and being coupled toone only of the said cavities and being mounted thereon with theelectrode at one end of its deceleration region electrically connectedto the inner conductor of the cavity and the electrode at the other endof its deceleration region electrically connected to the outer conductorof the cavity, means for applying a high-voltage power supply to each ofthe said accelerator-decelerator tubes, and modulator means having amodulation input connection and a plurality of outputs which areconnected to the modulator electrodes of the saidaccelerator-decelerator tubes, for producing a pulsed electron beamwithin each of the said accelerator-decelerator tubes, said electronbeam being pulsed at a repetition rate related to the resonant frequencyof the said resonant cavities, the said modulator means comprising meansfor causing a pulse position modulation of the pulses of the electronbeams so that there will be a difference between the timing of the beampulses in one of the said cavities and the timing of the beam pulses inanother of the said cavities, and for varying the magnitude of the saiddifference in response to signals applied to the said modulation inputconnection.
 11. A radio transmitter antenna as claimed in claim 10 andcomprising stabilization means, connected to each of the said cavitiesand responsive to the amplitude of radiofrequency oscillations inducedtherein by the electron beam pulses, and cOnnected to the modulatormeans, for controlling the energy of the electron beam pulses so as tostabilize the said amplitude.
 12. A radio transmitter antenna as claimedin claim 11 and wherein the said stabilization means comprises adetector means mounted in each of the said cavities for deriving afeedback signal indicative of the amplitude of radiofrequencyoscillations induced therein, and pulse-width-controlling meansconnected to receive the said feedback signal from the said detectingmeans and connected to the said modulator means for controlling theduration of the electron beam pulses.
 13. A radio transmitter antenna asclaimed in claim 12 and wherein the said detector means comprises asubsidiary collector electrode in each of the saidaccelerator-decelerator tubes.